1. Field of the Invention
The present invention relates to a light emitting diode (LED), and more particularly to an LED with high brightness.
2. Description of the Related Art
The LED is generally applied to the indication lamps of household appliances, the back light source of liquid crystal displays, browsers, third brake lights for cars, etc. Recently, AlGaInP and AlGaInN are well developed as the material of the LED. Therefore, the conventional incandescent lamp is replaced with the LED in various applications.
FIG. 1 and FIG. 2 are respectively the top view and cross-sectional view of a conventional LED 10, wherein FIG. 2 is along line 1-1 in FIG. 1. An N-type semiconductor layer 11, an active layer 16, a P-type semiconductor layer 12 and a transparent conductive layer 13 are sequentially formed on a substrate 17. Subsequently, an N-type electrode 14 and a P-type electrode 15 are formed after an etching step and a deposition step. The active layer 16 can be a homo junction, a hetero-junction, a double hetero-structure, a single quantum well, a multiple quantum well, etc.
Electrons generally move from the underside of the N-type electrode 14 to the P-type electrode 15, and meanwhile electronic holes move from the P-type electrode 15 to the N-type electrode 14. After the electrons and the holes are combined with each other at the active layer 16, light is emitted due to discharging energy. Either the electrons or the holes move along the shortest path between the P-type electrode 15 and the N-type electrode 14. Therefore, the combinations of the electrons and the holes concentrate in a local region of the active layer 16, but the majority of the active layer 16 is not used to generate light. Furthermore, generated heat concentrates at the same light region so that illumination and reliability are likely reduced.
To solve the aforesaid problems, LumiLeds Lighting Co. put forth an improved structure of parallel electrodes disclosed in U.S. Pat. No. 6,307,218, as shown in FIG. 3. Either P-type electrodes 35 or N-type electrodes 34 like comb-shaped (or finger-shaped) are provided on a LED 30. In comparison with FIG. 1, the comb-shaped electrodes substantially increase the passing paths and distribution areas of current; hence the illumination efficiency of an active layer is improved. In this regard, such electrodes are especially suitable for large-area and high-brightness LEDs. FIG. 4 shows a cross-sectional diagram along line 2-2 in FIG. 3. Similarly, an N-type semiconductor layer 31, an active layer 36 and a P-type semiconductor layer are sequentially deposited on a substrate 37. Subsequently, the P-type electrodes 35 and N-type electrodes 34 are formed through deposition and etching steps.
Apparently, the merits of comb-shaped electrodes are especially suitable for a large-area LED; however, the tooth corners of the electrodes can possibly cause electrostatic charges to abnormally discharge so that the partial circuit is burned therein. On the other hand, the fabricating processes and corresponding masks for the comb-shaped electrodes require a relatively high level of precision, and as a result high-performance equipment with low throughput is needed to fabricate them. Otherwise, the yield rate of the LED fabrication is quite reduced. The middle region of the LED has less thermal dissipation than the edge region. Accumulated heat in a certain area causes reductions in light efficiency and lifespan of the LED.
In conclusion, the optoelectronic market urgently requires a large-area LED having reliable and uncomplicated electrodes so as to overcome the aforementioned shortcomings of the conventional LED with high brightness.